Systems in a network environment communicate information in packets that encapsulate the information according to network communication protocols. Hosts using the Internet Protocol version 6 (IPv6) to communicate messages over a network can communicate over an Internet Protocol version 4 (IPv4) tunnel through a border router that encapsulates the IPv6 packets from the host within an IPv4 header as described in the publication “Transition Mechanisms for IPv6 Hosts and Routers”, Request for Comments (“RFC”) 2893 (Copyright 2000, The Internet Society). The border routers that transmit the packets over the IPv4 tunnel implement both the IPv4 and IPv6 protocols. For security reasons, when a router receives a tunneled packet, the router validates the source address in the IPv4 header to determine whether the packet originated from a router having a valid and trusted source address. A single IPv4 interface at a router can have numerous configured tunnels and the source address can be one of many source addresses.
A network processor in routers implementing IPv4 and IPv6 tunneling maintains in memory lists of valid possible source addresses to use to validate the source address included with the IPv6 over IPv4 transmission. Once the source address in the IPv4 header is verified, the receiving router may forward the decapulsated IPv6 packet to the IPv6 network and destination host.